When George and Nick are outside talking under the large tree, George pours a small amount of bourbon into Nick's glass. A second later, we see the glass has much more bourbon in it than was originally poured.
The first scene of George and Martha entering the living room shows a 3 light floor lamp in the corner of the room. The next scene of the area shows a single globe lamp hanging from the wall. The single globe lamp remains through out the film.
In the bedroom scene near the beginning, when Burton lies on the bed, the books behind his head are fully visible, but when Taylor lies down next to him a minute later, the books are half-obscured by the pillows.
The four characters stop at a bar after the first soiree at George's house. It is clearly after 2:00 a.m., since the time was stated during the first segment. No bars, however, would have been open after 1 or 2 a.m. in the New England states, where the film is set.
In almost every scene, to go up the stairs there are four or five steps and then the staircase turns left before continuing. Late in the movie, when Sandy Dennis enters through the front door, the staircase is straight with no turns. Further, if there were separate staircases, they almost certainly would not both go up from the front of the house.